Insulation for photoelectric apparatus



Nev. 11, 1941. METCALF 2,262,537

INSULATION FOR PHOTOELECTRIC APPARATUS Filed July 15, 1939 I /Z vv 2 a5 6 J k g 7 I F 25W Z2 Z3 43 5 L'J'\ 5% g TEMPERATURE Qr "6118.276 Z]? (DEGREE ,9 3 72w .5

Patented Nov. 11, 1941 INSULATION FOR PHOTOELECTRIC APPARATUS Arthur G. B. Metcalf, Milton, Mass, assignor to Photoswitch Incorporated, Cambridge, Mass, a corporation of Massachusetts Application July 15, 1939, Serial No. 284,638

4 Claims.

The operation of electric circuits incorporating photoelectric cells depends upon variations of the impedance of the cell which is very high when the cathode is not exposed to light, but decreases considerably upon illumination of the cathode; sensitive apparatus of this type has to respond to very slight changes in cell impedance. It has been observed that equipment of this type is very unreliable, and often inoperative when used in certain environments; for example, conventional photoelectric installations for supervising variations of smoke gas density, which must be highly sensitive and operate in temperatures from normal to as high as 200 F. and more, have been found to be very unreliable and could not be adjusted to operate within the required sensitivity range under all working conditions.

My investigation of this problem has revealed that this defect is due to changing impedance characteristics of such circuits, dependent upon ambient temperature changes, and that the changing temperature is able to efiect these impedance variations because conventional photoelectric apparatus employs, at points where they may unfavorably afiect the photocell impedance, elements whose impedance varies with the temperature. I have also found that it is not sufficient merely to provide at such points elements whose impedance does not break down at high temperatures, but that such elements should have a fairly constant temperature-impedance relation; an increase of impedance with increasing temperature would be just as detrimental as the more common decrease, even to a comparatively slight degree, of the impedance with increasing ambient temperature.

It is therefore the principal object of the invention to provide highly sensitive photoelectric equipment which retains this sensitivity to minute changes of effective light flux regardless of variations of ambient temperature whose range is greater than that which can be maintained under normal conditions. In on of its aspects,

the invention involves an arrangement substantially eliminating the above defect of those elements of such equipment which determine the impedance controlling its operation; the invention also contemplates an especially beneficial construction of the photoelectric cell and its socket, derived from the above-mentioned recognition of the cause of the failure of conventional equipment under the described conditions of varying ambient temperature.

invention will be apparent from the following description of an embodiment thereof which illustrates its genus by way of example, and refers to a drawing in which:

Fig. 1 is a circuit diagram of apparatus incorporating the invention;

Fig. 2 is a portion of the circuit according to Fig. l, with the critical impedances indicated;

Fig. 3 is a diagram of the temperature-impedance relations of certain materials; and

Fig. 4 is a view, partly in cross-section, of apparatus portion according to Fig. 2, incorporating the invention.

The circuit shown in Fig. 1 is characteristic of systems to which the present invention may be applied; it is at length described in Patent No. 2,154,480, of April 18, 1939, to E. R. Toporeck, and therefore will only be shortly explained herein. In such a circuit, a and b are the terminals of a source of supply, P is a photoelectric cell with anode I I and cathode I2, T is an amplifier tube with cathode 4, heater element 2, control electrode 6 and anode 3!. A comparatively high impedance I is connected on one side to grid and photocell, and on the other side to a supply terminal. The other terminal is connected to plate 3|, through relay coil M and condenser I parallel thereto. The various operating potentials are derived at 3, I4 and 22 from potentiometer I including the heater element 2 and connected between the two terminals. A relay switch S is actuated by coil M. A system of this type operates in the following manner, assuming that lines a, b are supplied with alternating current.

If photocell P is dark, and terminal a negative, plate 3I becomes positive. However, tap or slider 3 is so adjusted that the potential of cathode 4 is higher than the potential of grid 6 as controlled by impedance I; hence, since the control electrode 6 is sufiiciently negative relatively to cathode 4, current can not flow in magnet M, and switch S remains in normal position, for example open as shown.

If photocell P remains dark, and terminal a changes to positive, plate 3| will be connected to negative terminal 2;. Hence, tube T blocks current flow therethrough, magnet M remains deenergized, and switch S is not afiected.

If photocell P is now illuminated, and terminal a negative, plate 3| will be positive. The photocell whose impedance is now considerably reduced, will establish a current path from connection impedance I and grid 6 to line b, so that These and other objects and features of the the potential of control electrode 6 becomes more The circuit according to Fig. 1 will be employed if the relay is to be energized so long as light impinges on the photocell. trary, the switch is to be operated when the illumination of the photocell decreases, the connections are changed by tying photocell P to line a whereas resistor I is connected to tap M or directly to line b.

As pointed out in the above patent, correct operation and sensitivity of a circuit of this type depend upon the relation of the grid circuit impedances, namely that of impedance l on the one hand and of cell P on the other hand; for certain and exact operation, namely to give the same electrical response over a range of temperature variations, these impedances and their correlation should remain substantially unaffected by extraneous and uncontrollable influences.

As indicated in Fig. 2, the above critical relation can be affected at A through th connections to resistor I, at B through the connection to grid 6, and at C through the connections of both anode and cathode of photocell P. In this connection it should be noted that, according to operating conditions, either the cathode or the anode may be connected to grid 6. Any one of these connections may, through variation thereof, introduce a differential impedance I, P or 6' as indicated in Fig. 2, and it will be evident that such impedance changes, varying with some external characteristic as the ambient temperature, will make any preliminary setting and adjustment to maximumsensitivity more or less illusory.

As will be explained more in detail hereinafter, connection A can be maintained constant, for example by mounting resistor I in floating fashion, that is, directly suspended on the wires connecting its terminals to the heater element of amplifier tube T and to the cathode of photocell P, respectively.

Connections B and C necessarily involve seals leading into evacuated vessels, and hence the use of insulating material for separating conductors of different potential. Connections leading directly into the envelope involve only a single critical point, as for example indicated at B, whereas the use of base and socket connections involves two points where such material must be used, as indicated by the double bridging impedance at C of Fig. 2, one for the photo tube base and one for its socket.

I found that the impedance values of conventional bases and sockets made of plastic material vary considerably with changing temperature. Fig. 3 presents at m, n, three impedance-temperature curves for various molded or plastic materials conventionally used for that purpose; 'it will be evident that temperature changes of about 50, which occur quite frequently, may involve more than 50% impedance changes. For that reason, such material is totally unsuited for insulating purposes at the above points A, B and C.

I found, however, satisfactory materials whose If, to the con- I impedance remains substantially constant with changing temperatures. Such materials are, for example, mica, ceramic (glazed porcelain), or glass; the temperature-impedance relation of such materials is indicated in Fig. 3 at r, s, t, respectively, for the above examples.

A practical, actually used embodiment of the invention is shown in Fig. 4, which includes only the elements indicated in Fig. 2, which alone are essential for purposes of the invention.

In this figure, 20 is a mounting panel, for example of metal, 2| a socket element made of above referred to constant impedance material, as for example glazed porcelain, to which are fastened metal contact clips 22, 23. Inserted into the socket clips are prongs 24, 25 fastened in base 26 likewise made of constant impedance material, and fixed to the base in the usual way is envelope 2'! of photocell P, with cathode H and anode l2.

A conductor 28 leads through hole 29- of panel 20 to grid contact 3| connecting to amplifier tube T, which may be of the metal envelope type. Cap 32 and grid lead wire 33 are separated from the tube proper by means of constant impedance insulators 34, 35, respectively. It should be noted that a tube of this type is preferable to a glass tube even with top electrode connection, because the glass used for tubes of this type may not be a constant impedance type material, as above mentioned.

In the embodiment described, wire 4| is fastened to connector 43 of heater 2 of tube P, and wire 42 to clip 22 of the photocell socket. Freely suspended on these wires is impedance I.

In the embodiment shown, the resistance of the photocell, if dark, is of the order of about 500 megohms. Hence, for satisfactory operation, the resistance across the terminals of the base should be much higher than that; yet commonly used moulded plastic bases (which have been tested pursuant to my investigation of the problem of instability of photoelectric systems) may have a value of less than 200 megohms at a temperature of E, which is not at all unusual in industrial applications of photoelectric equipment. Moreover, as above explained, such materials change their impedance rapidly with varying temperature, so that the operation of the circuit would sometimes be controlled by temperature changes rather than illumination changes. The operation became satisfactory as soon as the cause of the defective operation, namely the temperature dependent impedance values, was discovered and constant impedance connections introduced.

top grid leads for th amplifier tube, base and socket connections as shown for the photocell may be used. I found that the connections of the photoelectric cell are especially critical and that it is of primary importance to use constant impedance material for photocell base and socket; the unsatisfactory operation is often rendered satisfactory if only this special precaution is taken.

It should be understood that the present disclosureis for the purpose of illustration only and that this invention includes all modifications and equivalents which fall within the scope of the appended claims.

I claim:

1. In combination with highly sensitive photoelectric equipment for operation under ambient temperature conditions varying within a range greater than normal temperature changes, a circuit element comprising a control impedance, an amplifying tube including a control electrode, a photoelectric cell with two electrodes, and electric connections having a substantially constant temperature-impedance relation between an electrode of said cell, said control electrode and said control impedance under said varying temperature conditions.

2. In combination with highly sensitive photoelectric equipment for operation under ambient temperature conditions varying within a range greater than normal temperature changes, a circuit element comprising a mounting support, a control impedance, an amplifying tube including a control electrode, a photoelectric cell having two electrodes, a base having a substantially constant temperature-impedance relation and electric connectors leading to said electrodes and secured in said base, a socket having a constant temperature-impedance relation mounted on said support and having contacts receiving said connectors, and connections having a temperature-impedance relation between one of said contacts, said control electrode and said control impedance which is under said varying temperature conditions substantially constant.

3. In combination with highly sensitive photoelectric equipment for operation under ambient temperature conditions varying within a range greater than normal temperature changes, a support, mounted on said support a socket having two contacts, a photoelectric cell with cathode and anode and a base with two electric connectors leading to said cathode and said anode, respectively, and engaging said contacts, the impedances of said socket and said base between said contacts and said connectors, respectively, being substantially constant with said varying temperature conditions.

4. In combination with highly sensitive photoelectric equipment for operation under ambient temperatur conditions varying within a range greater than normal temperature changes, a support, mounted on said support a socket having two contacts, a photoelectric cell with cathode and anode and a base with two electric connectors leading to said cathode and said anode, respectively, and engaging said contacts, said base and said socket being made of material having substantially constant impedance at said varying temperature conditions.

ARTHUR G. B. ME'ICALF. 

